1. Field of the Invention
The present invention relates to a novel graft copolymer, a method for preparing the same and a thermoplastic resin composition comprising the same, more particularly to a graft copolymer having superior impact resistance, weathering resistance, gloss and pigmentation property, which is prepared using a composite rubber polymer prepared by particle enlargement after mixing a diene rubber latex and an alkyl acrylate rubber latex, a method for preparing the same and a thermoplastic resin composition comprising the same.
2. Description of Related Art
ABS resin, an acrylonitrile-butadiene-styrene three-component copolymer, is widely used in a variety of fields, including electric, electronic, construction and automobile fields, because of superior impact resistance, rigidity, chemical resistance and processing property. However, ABS resin has weak weathering resistance because it comprises a butadiene polymer, and thus it is inadequate for outdoor use.
To obtain a thermoplastic resin with superior physical properties and good weathering resistance and aging resistance, there should be no ethylenic unsaturated polymer in the graft copolymer. ASA resin (acrylonitrile-styrene-acrylate three-component copolymer) using a cross-linked alkyl acrylate rubber polymer has been confirmed to be a thermoplastic resin having supeiror weathering resistance and aging resistance. Because of superior weathering resistance and aging resistance, the ASA resin is used in a variety of fields, including automobiles, ships, leisure goods, construction materials and gardenings.
German Patent No. 1,260,135 disclosed a method for preparing an ASA polymer having superior weathering resistance and aging resistance in which a large-diameter latex of cross-linked acrylate having a narrow particle size distribution having an average core particle diameter of 150-800 nm was used. Compared with a polymer prepared using a small-diameter polyacrylate latex, a polymer comprising a large-diameter polyacrylate latex has improved notch impact strength and experiences less contraction. However, the large-diameter graft copolymer has the problem that it is not colored as easily as the small-diameter graft copolymer. Thus, use of the ASA polymer is limited when producing a colored product, giving dull-colored product rather than bright one.
U.S. Pat. No. 4,224,419 disclosed a easily colored thermoplastic resin having good weathering resistance and impact resistance comprising a first graft copolymer prepared from a cross-linked acrylate polymer having an average particle diameter of about 50-150 nm, as core, and styrene and acrylonitrile, as graft shell, a second graft copolymer prepared from a cross-linked acrylate polymer having an average particle diameter of about 200-500 nm, as core, and styrene and acrylonitrile, as graft shell, and a hard component comprising a copolymer of styrene and/or α-methylstyrene with acrylonitrile and, the weight ratio of the core constituents being from about 90:10 to 35:65 and the proportion of the sum of the two core constituents being from about 10-35 wt % based on the total mixture.
European Patent No. 534,212 and U.S. Pat. No. 5,932,655 each disclosed a method of improving pigmentation property and impact resistance by preparing large-diameter and small-diameter graft copolymers and a hard component having a glass transition temperature higher than room temperature, as seed component. European Patent No. 534,212 used the hard seed only in the small-diameter graft copolymer and U.S. Pat. No. 5,932,655 used the hard polystyrene seed in both the large-diameter and the small-diameter graft copolymers.
These materials offer superior weathering resistance and mechanical property with improved pigmentation property. However, the improvement of pigmentation property is not sufficient, as yet. These materials use a cross-linked alkyl acrylate polymer as rubber constituent, which has too small refractive index, compared with styrene or acrylonitrile, to offer vivid color. Further, because the cross-linked alkyl acrylate polymer has a high glass transition temperature, impact strength is not sufficiently high at low temperature.
To overcome these problems, Japanese Patent Laid-Open No. Sho 59-49245 and U.S. Pat. Nos. 4,912,162 and 4,393,172 disclosed methods of using multi-layered alkyl acrylate rubber polymers containing diene rubber polymers. However, because inside layer of the diene polymer is not sufficiently covered by the outside layer of the alkyl acrylate polymer, it is difficult to obtain a resin having superior weathering resistance. Also, cross-linking at the inside layer of the diene polymer during formation of the outside layer of the alkyl acrylate polymer tends to reduce such properties as impact strength.